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INTRODUCTORY   ESSAY.                                171
parts of India, an upper current of south-westerly wind
carries its moisture to the higher mountains, where it is con-
densed in the form of snow. Snow falls in the eastern parts,
in severe seasons, as low as 5000 feet, and in the north-
west occasionally as low as 2000 feet. The ordinary limit,
however, is several thousand feet higher. After the vernal
equinox, by which time the south-west monsoon has fairly
set in, the sky is usually serene and the weather beautiful.
To the eastward this rule is subject to frequent exceptions,
the same causes which make the climate of Bengal humid
at all seasons operating more markedly on the Himalaya to
the northward of that province. As summer advanced, the
wind becomes more humid, and occasional' heavy thunder-
storms in the afternoons mark the approach of the rains-,
which set in about midsummer; considerably earlier, however,
in the eastern than in the north-western Himalaya. During
the rainy season, which continues almost till the autumnal
equinox, when the decreasing declination of the sun changes
the direction of the wind, the atmosphere is very humid, usu-
ally almost to saturation. There are, however, occasional in-
terruptions in the rains, during which the weather is superb.
The rain-fall is greatest to the eastward, and diminishes gra-
dually in advancing westward.
As the source of the deluge of rain w;hich falls on the Hima-
laya is very distant, a great part of the moisture is necessa-
rily deposited on the first range with which the humid .wind
comes in contact, of sufficient elevation to cool the air to the
point of saturation. The ram-fall'is therefore greatest on
ranges elevated from 6 to 10,000 feet, especially where these
advance in considerable masses near to the plains, while
isolated peaks, and ranges of lesser elevation, as well as the
valleys of the great rivers, are evidently drier. As a conse-
quence of this, all the valleys of the interior which -are sepa-
rated from the plains by continuous chains, attaining an ele-
vation of 10-12,000 feet, are to a great extent sheltered by
these from the rains, which fall only as occasional showers;